U.S. patent number 4,141,648 [Application Number 05/750,936] was granted by the patent office on 1979-02-27 for photoconductor charging technique.
This patent grant is currently assigned to International Business Machines Corporation. Invention is credited to Ronald E. Gaitten, Gerald L. Smith.
United States Patent |
4,141,648 |
Gaitten , et al. |
February 27, 1979 |
**Please see images for:
( Certificate of Correction ) ** |
Photoconductor charging technique
Abstract
A two cycle process electrophotographic copying device having
charging, imaging, developing, transferring, and cleaning
facilities, the arrangement being in the conventional sense,
incorporates a combined charge and preclean corona unit that is
operable to perform either a charging function or a precleaning
function at the proper time during a copying/cleaning cycle and a
combined precharge/transfer corona unit. The combined charge and
preclean corona unit includes a dual bay corona which emits
negative and positive ions and a common control grid for
controlling the ion flow from each bay of the dual bay corona.
Inventors: |
Gaitten; Ronald E. (Boulder,
CO), Smith; Gerald L. (Broomfield, CO) |
Assignee: |
International Business Machines
Corporation (Armonk, NY)
|
Family
ID: |
25019750 |
Appl.
No.: |
05/750,936 |
Filed: |
December 15, 1976 |
Current U.S.
Class: |
399/148; 250/325;
361/229; 399/171; 399/50; 399/89 |
Current CPC
Class: |
G03G
15/0291 (20130101); G03G 21/06 (20130101) |
Current International
Class: |
G03G
21/06 (20060101); G03G 15/02 (20060101); G03G
015/14 (); H01T 019/04 (); G03G 021/00 () |
Field of
Search: |
;355/3R,3CH,3DD,14,15
;250/325,326 ;361/229,235 |
References Cited
[Referenced By]
U.S. Patent Documents
Other References
Noto, Fedele A., "Copier with Single Corona Generating Device",
Xerox Disclosure Journal, vol. 1, No. 2, Feb. 1976, p. 93..
|
Primary Examiner: Braun; Fred L.
Attorney, Agent or Firm: Cockburn; Joscelyn G.
Claims
What is claimed is:
1. In a two cycle process, two corona electrophotographic machine
where the photoconductor is being charged, imaged, toned,
transferred and cleaned by stations positioned relative to said
photoconductor, the improvement comprising in combination:
a combined charging and transfer station for charging the
photoconductor and for charging a transfer means; and
a combined overcharging and smoothing station; said station having
a dual bay corona with one bay for overcharging the photoconductor
and a second bay for smoothing and reducing the overcharge on said
corona;
a common grid having a voltage level associated with the bays of
the combined charging and smoothing station; and
control means connected to the common grid and operable for
switching the voltage level on the common grid so as to perform the
cleaning or the charging of the photoconductor.
2. In a two cycle process electrophotographic apparatus having a
photoconductor with the customary facilities to charge, image,
develop, transfer and clean the improvement comprising:
a first charging means operable for charging the photoconductor
surface to a first voltage level during the first cycle and for
charging a transfer media to a second voltage level during the
second cycle;
a second charging means positioned downstream of said first
charging means; said second charging means operable for charging
the photoconductor surface with a first controlled voltage and a
second controlled voltage for smoothing the charge on said
photoconductor and a third controlled voltage for precleaning the
photoconductor; and
a control grid means, associated with said said second charging
means, operable to enable the charging, and the smoothing of the
charge on the photoconductor and to enable the precleaning of the
photoconductor.
3. The device as claimed in claim 2 where the second charging means
includes:
a dual bay corona;
said dual bay corona having a first corona bay for generating ions
with a first polarity;
a second corona bay for generating ions with a polarity opposite to
the polarity of the ions being generated by the first bay;
first power supply means operably connected to the first corona
bay; and
second power supply means connected to the second corona bay.
4. The device as claimed in claim 2 further including switching
means connected to the grid and operable for switching the grid to
different voltage levels to effectuate cleaning or charging the
surface of the photoconductor.
5. The device as claimed in claim 2, further including a common
power supply connected to the first charging means and the second
charging means.
6. The device as claimed in claim 2 wherein the magnitude of the
first voltage level is less than the voltage level at which the
photoconductor surface is imaged.
7. The device as claimed in claim 2 wherein the first controlled
voltage has a polarity identical to the polarity of the first
voltage level and operable to overcharge the surface
photoconductor.
8. An improved two corona charging system for use with a two cycle
process electrophotographic machine comprising:
a combined precharge and transfer corona, said corona being
operable during the first cycle to charge a photoconductor to a
first polarity and being operable during the second cycle to charge
a transfer means;
a dual bay corona positioned downstream from the combined precharge
and transfer corona; said dual bay corona having a first bay for
depositing a charge, with a polarity substantially equivalent to
the first polarity, on the photoconductor and a second bay for
depositing a charge with opposite polarity for smoothing and
reducing the charge on said photoconductor;
a common grid structure associated with the dual bay corona and
operable to control the deposition of charge on the
photoconductor;
control means operably connected to the grid structure;
power supply means operably connected to the coronas;
imaging means being positioned downstream from the dual bay corona
and for creating an electrostatic image on the photoconductor;
developing means being positioned downstream from the imaging means
for depositing toner particles on the electrostatic image and for
cleaning the photoconductor; and
transfer means associated with the photoconductor, for transferring
the toned electrostatic image.
9. The device claimed in claim 8 wherein the combined precharge and
transfer corona and one of the bays of the dual bay corona are
connected to a common power supply means. pg,29
10. An improved two corona charging system for use with a two cycle
process electrophotographic machine comprising:
a combined precharge and transfer corona, said corona being
operable during the first cycle to charge a photoconductor to a
first polarity and being operable during the second cycle to charge
a transfer means;
a dual bay corona positioned downstream from the combined precharge
and transfer corona; said dual bay corona having a first bay for
depositing a charge, with a polarity substantially equivalent to
the first polarity, on the photoconductor and a second bay for
depositing a charge with opposite polarity for smoothing and
reducing the charge on said photoconductor;
common grid structure associated with the dual bay corona;
control means operably connected to the grid structure;
power supply means operably connected to the coronas;
an intererase means for discharging the border areas of the
photoconductor during the first cycle of the two cycle process and
for discharging the entire surface area of the photoconductor
during the second cycle of the two cycle process;
imaging means being positioned downstream from the dual bay corona
and for creating an electrostatic image on the photoconductor;
11. In a two cycle process electrophotographic machine having a
photoconductor surface with the customary facilities to charge,
image, develop, transfer and clean, an improved charging system for
said machine comprising:
a first corona for charging a transferring means to transfer a
latent image from the photoconductor surface;
a dual bay corona positioned downstream from the first corona and
operable for charging the photoconductor surface;
said dual bay corona includes a first bay for depositing a first
charge having a potential which is in excess of the operating
potential of said photoconductor and a second bay for depositing a
second charge for smoothing and reducing the first charge on said
photoconductor;
a common grid structure associated with the dual bay corona and
operable to control electron flow from the dual bay corona to
charge the surface of the photoconductor; and
switching means connected to the common grid structure;
said switching means being operable during the first cycle to a
first voltage level on the common grid structure and to set a
second voltage level on the common grid structure during the second
cycle of said two cycle process whereby the first bay of the dual
bay corona has negligible charging effect on the surface of the
photoconductor during the second cycle to enable precleaning of the
photoconductor.
12. The device claimed in claim 11 wherein the first voltage level
on the grid structure is substantially equivalent to -1000
volts.
13. The device claimed in claim 12 wherein the second voltage level
on the grid structure is 0 volts.
Description
RELATED APPLICATION
The copending application of Gerald Lee Smith, Ser. No. 750,800
filed Dec. 15, 1976 and commonly assigned is incorporated herein by
reference. The application of Gerald Lee Smith is a
continuation-in-part of application Ser. No. 580,643, filed May 27,
1975, now abandoned.
BACKGROUND OF THE INVENTION
1. Field of the Invention
The invention relates to an electrophotographic copying device and,
more specifically, to an improvement over the charging and cleaning
of the support surface, also known as photoconductor on which the
latent image of an original is developed.
2. Prior Art
The following U.S. Patents are representative of the prior art:
U.S. Pat. Nos. 3,647,293; 3,637,306; and 3,736,055.
Numerous prior art teachings in the field of electrophotographic
copying teaches various methods and devices for preparing or
charging the surface of a photoconductor so as to obtain a latent
image from an original document. Prints are then transferred from
the latent image on the surface of the photoconductor, to a
transferring media.
To enable the development of the latent image on the photoconductor
and the transferring of said latent image to a transferring media,
several stations are arranged in proximity to and to cooperate with
the photoconductor to perform certain functions. At the charging
station, the photoconductor is charged to a selective polarity, be
it positive or negative. The photoconductor then moves to the
exposing or imaging station where a latent image is copied from the
original document. Next, the electrostatic latent image is
developed at a developer station to form a toned image on the
photoconductor. The toned image is then transferred from the
photoconductor to another media at the transferring station. To
complete the cycle, the photoconductor is erased, precleaned, and
cleaned and is then ready for another cycle.
Although the prior art electrophotographic devices function
adequately for the intended purpose, several problems plague these
systems.
Probably one of the most pressing problems is the fact that the
charging, transferring and precleaning functions are all performed
by separate coronas at separate stations. These systems are
referred to as a three corona type system. With this type of prior
art design, the cost of the electrophotographic device is
relatively high, due to the individual cost of each corona. Since
the general trend is to minimize the cost of electrophotographic
devices without sacrificing efficiency or copy quality, any
reduction in the number of component counts, in the prior art
devices, will be welcome.
Another problem relating to the separate processing station is the
fact that each of the separate coronas require a separate power
supply. The aggregate cost of these power supplies further augments
the overall cost of the unit. As such, any reduction in the number
of power supplies will reduce the cost of the unit.
It is common knowledge that conventional electrophotographic
devices may be either a single cycle process or a two cycle
process. In the typical two cycle process the photoconductor is
charged, imaged and developed during the first cycle while the
image is transferred and the photoconductor is cleaned in the
second cycle. For satisfactory operation, some of the stations
which render necessary functions during the copying process are
active during the first cycle, while others are inactive and vice
versa. On account of the rapid speed at which the photoconductor
accesses each of the stations, it is, therefore, necessary for high
speed switching to occur at these stations. The conventional 60
cycle power supply which is used for supplying power to these
stations cannot withstand high speed switching. With these
drawbacks, it is clear that a more efficient device which utilizes
a more efficient charging technique is needed.
Several attempts have been made to improve the prior art
electrophotographic devices by solving some of the above identified
problems. For example, attempts have been made to combine the
charge and the transfer corona station. At first blush, this
combination seems to be workable and logical since the function of
both stations is to supply charges having a given polarity.
However, the combination, instead of solving the above described
problems, creates additional problems.
One of the additional problems stems from the fact that the
combined charge transfer station is designed with a grid structure
to enhance the charge operation. However, transferring media which
is fed into the machine at the charge/transfer station for
transferring the latent image from the photoconductor jams into the
grid wires. This jam results in machine breakdown.
For proper operation, if the charge on the toned image is positive,
a negative charge has to be deposited onto the transferring media
so that the positively charged toner particles will be attracted.
Of course, if the toner is negatively charged then the transferring
media has to be charged positively. With the presence of the grid
assembly in the combined charge/transfer station, the charge
(negative or positive) cannot be uniformly distributed onto the
transfer media. With an uneven distribution of charges, the quality
of the final copy is less than satisfactory.
OBJECTS OF THE INVENTION
It is, therefore, the object of the invention to design a more
efficient, low cost electrophotographic device than was heretofor
possible.
It is a further object of this invention to combine the preclean
and charge coronas into a single unit. The single unit incorporates
a dual bay corona with one bay for overcharging and another bay for
reducing and/or smoothing the charge.
It is still a further object of the present invention to use the
transfer corona station to render some of the precharging and the
transferring functions.
It is still another object of the invention to utilize the combined
precharge and transfer corona to render either the precharging of
the photoconductor or the charging of the transfer media with said
corona being set at one current level.
It is still another object of the invention to provide a corona
configuration that is usable in a two cycle process but does not
require power supply switching (except for the grid supply).
It is still another object of the invention to provide a corona
configuration that will develop a smoother charge distribution on
the photoconductor than was possible.
SUMMARY OF THE INVENTION
The present invention overcomes the aforementioned drawbacks in the
prior art by means of a unique structural combination of processing
stations within the copying process. More specifically, the
combination allows the use of a two corona system for charging
and/or precleaning the photoconductor and for charging the
transferring media in a two cycle process. In one feature of the
invention, during the first cycle, the photoconductor is precharged
to a first potential which may be more or less than its operating
potential. The charging is performed by the combined precharge
transfer corona (called a Corotron): The charge or first potential
is then augmented to an overcharge by one bay of a dual bay corona
which forms the combined charge/preclean corona (called a
Scorotron). The other bay of the dual bay corona then reduces and
smoothes the overcharge of the photoconductor to the operating
potential of said photoconductor.
The interimage erase lamp discharges the border area and/or no copy
area of the photoconductor.
Imaging and developing also occurs during this first cycle.
During the overcharging and smoothing of the photoconductor the
dual bay corona is controlled by a single gridded structure.
During the second cycle of the two cycle process, the toned image
is transferred to the transferring media using the same precharge
transfer corona (Corotron). Following transfer, the drum is charged
and/or discharged by the charge/preclean corona to a second
potential for cleaning. In order to place the second charge or
potential on the photoconductor, the grid of the charge/preclean
corona is switched to a different voltage level which may be of the
opposite polarity. Of course, the setting is chosen so as to obtain
best cleaning. The photoconductor is then (optionally) erased by
the interimage erase lamp, and cleaned by the developer/cleaner
station.
In another feature of the invention, power is supplied to the
precharge transfer corona and one bay of the dual bay corona by a
common power supply.
In still another feature of the invention, both the overcharging
and the smoothing function is achieved by the dual bay corona. In
this configuration, the precharge transfer corona is used for only
charging the transfer media.
The foregoing and other objects, features and advantages of the
invention will be apparent from the following more particular
description of the preferred embodiment of the invention, as
illustrated in the accompanying drawings.
BRIEF DESCRIPTION OF THE DRAWINGS
FIG. 1 shows the photoconductor of an electrophotographic machine
with a plurality of processing stations, positioned relative
thereto and incorporates the present invention.
FIGS. 2 and 4 are a schematic diagram of a two cycle process
electrophotographic machine showing a plurality of processing
stations which is helpful in understanding the present
invention.
FIG. 3 is a schematic diagram showing one embodiment of a control
circuit which changes the voltage of the control grid in the
combined charge/preclean station.
DETAILED DESCRIPTION
The term corotron as used in this application means a type of
corona having either limited or no grid structure. In effect the
corotron may be considered analogous to a current source.
The term scorotron as used in this application means a type of
corona having a grid structure. The scorotron may be considered as
a voltage source.
The term corona as used in the application is generic for corotron
and/or scorotron.
For exaplanation purposes, the photoconductor in the preferred
embodiment of the present invention will be described as a rotating
drum. However, this should not be construed as a limitation on the
scope of the invention; since it is well known in the art to design
a photoconductor having a different shape, size and mechanical
configuration. For example, the photoconductor may be a continuous
belt or a plate rather than a rotating drum structure.
Although the preferred embodiment of the invention is described in
association with a two cycle copying process, this should be
interpreted as illustrative rather than restrictive, since it would
be obvious for one skilled in the art to modify the inventive
feature as disclosed hereinafter to make said concept operable in a
one cycle copying process. In the drawings, similar elements are
identified by identical numbers.
Referring now to FIGS. 1, 2 and 4, a schematic of an
electrophotographic copying system 10 is shown. The end view of a
cylindrical drum 12, hereinafter called the photoconductor, is
mounted for rotation on shaft 14 and having on its outer periphery
a photoconductive insulating layer which contains an organic or
inorganic photoconductive material. The drum 12 is rotated to bring
the photoconductive layer in spaced relationship with various
stations associated with the electrophotographic process; each of
said stations being positioned in proximity to the rotating
drum.
A negative corotron 18 is positioned within the orbit of
cylindrical drum 12 to define the so called precharge/transfer
station 32. Negative corotron 18 of the precharge/transfer station
serves two functions, namely: to deposit ions on the surface of the
photoconductor, and to deposit ions on a transferring media, for
example, paper which is introduced to the precharge/transfer
station at paper path 62.
The polarity of the charge on the ions may be positive or negative
depending on the type of electrophotographic system. In the
preferred embodiment of this invention, the charge is negative. As
will be explained subsequently, the negative charge which is
deposited on the photoconductor by negative corotron 18 is rough;
i.e., the charge is unevenly distributed on the surface of the
photoconductor. Also, the same current level setting may be used
for charging the photoconductor and for charging the transfer media
which avoids the need to switch the current level of the
precharge/transfer corona.
Although the magnitude of the charge may vary, it has been found
and practiced in the preferred embodiment of the invention that the
photoconductor is more uniformly charged when the magnitude of the
charge, which is deposited on the photoconductor by the
precharge/transfer corona, is less than the magnitude of the charge
which is necessary for proper operation of the photoconductor. For
example, the charge which is deposited on the photoconductor is
approximately -800 volts while the operating voltage of the
photoconductor is approximately -870 volts.
After precharge/transfer station 32, the next station in order is
the combined final charge/preclean station 20, also referred to as
the dual bay corona. Final charge/preclean station 20 is the
facility which supplies ions (positive and/or negative) to
overcharge the surface of the photoconductor in the first bay and
to final charge the surface of the photoconductor in the second bay
(with the opposite polarity charge) and renders the preclean
function during the second cycle. As used in this application,
overcharge means that the photoconductor is charged to a potential
beyond its operating potential. This photoconductor charge is
referred to as smooth due to the fact that the charge is evenly
distributed over the surface of the photoconductor because of the
cutoff characteristic produced by the control grid 24. As will be
explained subsequently, the polarity of the emission wires in the
first bay of the final charge/preclean corona is the same as the
voltage applied by the precharge/transfer corona, so that a second
stage of precharge is achieved that produces a smooth
photoconductor charge because of the grid. In the preferred
embodiment, a negative emission voltage is used in the first bay so
that negative ions are generated. The polarity of the emission
wires in the second bay is opposite to the polarity of the first
bay, while the grid of the second bay is electrically common to the
first bay and, therefore, has the same voltage. In the preferred
embodiment, a positive emission voltage is used in the second bay
so that positive ions are generated. This bay, therefore, charges
the photoconductor in the positive direction from the overcharge
level to the final desired dark charge level.
Still referring to FIGS. 1, 2 and 4, final charge/preclean station
20 comprises a dual bay corona which includes a trailing bay 22 and
a leading bay 104. The trailing bay is sometimes referred to as a
positive scorotron while the leading bay is called a negative
scorotron. The positive scorotron supplies positive ions or charge
while the negative scorotron supplies negative ions or charge. The
negative ion which is supplied by the negative scorotron augments
or increases the charge which is placed on the photoconductor by
the combined precharge/transfer station. Stated another way, the
photoconductor is overcharged to a given polarity by the negative
scorotron. In the preferred embodiment, the photoconductor is
overcharged to approximately -1100 volts. Likewise, the positive
ions or charge which is supplied by the positive scorotron smooths
or reduces the rough charge on the photoconductor to a voltage
level at which the photoconductor is functional. In the preferred
embodiments, the operating voltage of the photoconductor is
approximately -870 volts.
The dual bay corona which overcharges and smooths the voltage on
the photoconductor can take several structural design forms. For
example, in the preferred embodiment of this invention, shown in
FIGS. 1, 2 and 3, common corona case 104 defines both the leading
negative scorotron and the trailing positive scorotron. The common
case is interconnected to a common potential, for example, ground
by conductor 100. Another alternative design which is contemplated
by the present invention is a separate corona housing with separate
cases defining the negative and positive scorotron.
Referring again to FIGS. 1, 2 and 3, common grid structure 24 is
positioned between dual bay corona 20 and photoconductor 12. The
common grid controls the flow of ions from the dual bay corona so
as to deposit a uniform charge on the surface of photoconductor 12.
For proper operation of the system, the dual corona bays must be
controlled by a common grid structure. Again, the common grid may
take various design forms. For example, a single set of grid wires,
as is shown in the drawings, or separate sets of grid wires, one
for each corona bay. In the design where separate sets of grid
wires are used, then both grids must be connected to a common
control means.
As will be explained subsequently, switching circuit 36 (shown in
block diagram form in FIG. 1 and in more detail in FIG. 3) is
connected to grid 24 to control the voltage on the grid. A
plurality of grid control circuits can be generated without
departing from the scope of the present invention. The voltage on
the grid changes depending on whether the photoconductor is cleaned
or charged. For example, in one instance the voltage on the grid is
very negative (approximately -1000 volts), while in another
instance the grid is positive (for example +200 or +400 volts).
Still in another instance, the voltage is slightly negative and/or
positive (approximately .+-. 50 volts) or ground.
Positioned downstream from the combined charge/preclean station is
interimage station 26. Interimage station 26 incorporates
interimage erase lamp 28. The erase lamp cleans or discharges the
border area or no copy area of the photoconductor during the first
cycle of the two cycle process and cleans or discharges the entire
surface area of the photoconductor during the second cycle. The use
of the erase lamp during the second cycle is optional.
The next station in order is the image station 30. Image station 30
comprises a conventional optical system which functions to transfer
an image of a document onto the photoconductor. With the image on
the photoconductor, the next station in line is the developer
cleaner station 60. Developer cleaner station 60 is conventional.
For example, the developer cleaner station is analogous to the
developer cleaner station as disclosed in the above identified U.S.
Pat. No. 3,637,306, entitled "Copying System Featuring Alternate
Developing and Cleaning of Successive Image Areas for
Photoconductor" and assigned to the same assignee of the present
invention.
Referring now to FIG. 3, one embodiment of a control means which
controls the negative corotron 18 of precharged/transferred station
32 is disclosed. Also, an embodiment of control means 36 which
switches the polarity of grid structure 24 from a first potential
to a second potential is disclosed.
As was mentioned previously, negative corotron 18 of
precharge/transfer station 32 supplies negative ions to the
photoconductor in one cycle and in another cycle supplies negative
ions to a transfer medium which accesses corotron 18 along paper
path 62. In order to supply negative ions, a negative high voltage
power supply 34, also called control means 34, is connected to
corotron 18. Power supply 34 is set at a single current level for
both charging the photoconductor and for charging the transfer
media. As a result, a conventional 60 cycle power supply is used
since current level does not have to be switched.
In an alternative embodiment of the invention, corotron 18 and the
negative bay of the dual bay corona is interconnected to common
voltage supply means 110 (FIG. 1).
Still referring to FIG. 3, grid structure 24, also called control
means 24, functions as a limiting means for controlling the ions
(positive and/or negative charge) which are deposited on the
surface of photoconductor 12 from positive scorotron 22 and
negative scorotron 104. The resulting photoconductor voltage is a
function of the grid voltage. In order to effectuate this limiting
or controlling effect, a switching means is operably connected to
the grid for switching its voltage between two (or more) voltage
levels.
Switching means 36 comprises a diode 38 hereinafter called
unidirectional device 38. One terminal of the unidirectional device
is connected to grid 24 while the other terminal is connected to
positive terminal 40 hereinafter called third reference voltage
source 40. Third reference voltage source 40 may be any positive
value, negative value or ground. For example, in the preferred
embodiment of this invention the value is ground.
Resistor 48, hereinafter called third resistor means 48, connects
third reference voltage source 40 to a lower or equal potential. In
the preferred embodiment of this invention, the low potential is
ground. Likewise, another resistor 44, hereinafter called second
resistor means 44, connects third reference voltage means 40 to a
higher potential. In the preferred embodiment of the invention, the
higher potential is chosen to be 120 volts.
In an alternate embodiment of the invention, third reference
voltage source 40 is connected to a switchable preclean level
supply. The preclean level supply can be adjusted to one of a
plurality of voltage potentials. For example, typical voltage
levels would be +400 volts to -400 volts or ground.
Reference voltage source 46 hereinafter called first reference
voltage source 46 is positioned in parallel with third reference
voltage source 40. The potential of first reference voltage source
46 is negative. In the preferred embodiment of this invention, a
1000 volt negative potential is chosen. First reference voltage
source 46 is established by a conventional bank of neon tubes 48.
Of course, it would be obvious to one skilled in the art to
substitute conventional devices to establish first reference
voltage source 46 without departing from the scope of this
invention.
Resistor 50 hereinafter called first resistor means 50 is connected
in series with first reference voltage source 46 so as to establish
second voltage source 52. In the preferred embodiment of this
invention, source 52 is chosen to be 1500 volts negative. In an
alternate embodiment, second voltage source 52 is connected to a
negative grid supply means. The negative grid supply means has a
typical value of approximately -1500 volts. Switching means 54
interconnects unidirectional device 38 and first reference voltage
source 46. The connection is such that by activating switching
means 54 either the voltage at third reference voltage source 40 or
the voltage at first reference voltage means 46 is rendered
operative. Of course, several conventional switching devices may be
used for switching means 54. However, in the preferred embodiment
of this invention, switching means 54 was a high voltage read relay
switch. Positive high voltage supply 57 supplies power to scorotron
22 via terminal 56. In the preferred embodiment, high voltage
corona supplies 34 and 57 are current regulated so that they
deliver a constant total current to the corona emission wires. This
completes the detailed description of the preferred embodiment of
the invention.
Although the above invention is described in relationship with a
two corona charging scheme for an electrophotographic machine, the
invention can be extended to cover a three corona charging scheme.
In the three corona charging scheme application, the
precharge/transfer corona is one of the coronas, while the negative
bay of the dual bay corona and the positive bay of the dual bay
corona are the other coronas.
OPERATION
In order to simplify the operation of the two cycle process, which
embodies the present invention, the position of the processing
station in relation with rotating cylindrical drum 12 is equated
with positions on the face of a clock (see FIGS. 2 and 4). In
operation, cylindrical drum 12 rotates in the direction shown by
arrow 16. During the first cycle of the two cycle process (FIG. 2),
step 1A occurs at 6:00. At 6:00, the precharge/transfer constant
current negative corona 18 of precharge/transfer station 32
precharges the photoconductor of cylindrical drum 12 to a rough
negative voltage. For example, the undercharge voltage is -800
volts.
The second step, 1B, occurs at 11:00 where the small negative
leading bay (scorotron 104) of final charge/preclean station 20
augments the photoconductor charge to approximately -1100 volts as
controlled by grid 24 which has a setting of approximately -1000
volts. The overcharge. which is now on the photoconductor surface
is reduced or smoothed to approximately -870 volts by the large
positive trailing bay (scorotron 22) of the final charge preclean
station. At 12:00, step 2 occurs; lamp 28 of interimage station 26
performs the interimage erase. At 1:00, step 3occurs; the
photoconductor is imaged at image station 30, such that the
photoconductor charge in a black image is approximately -840 volts,
the photoconductor charge in a gray image is approximately -400
volts, and the erase background and white charge is from -150 to
-180 volts.
At approximately 4:00, step 4 occurs; the latent image is developed
by magnetic brush 58 of developer/cleaner station 60. The bias of
magnetic brush 58 is approximately -350 volts. Thus, magnetic brush
58 is positive relative to the latent image and negative relative
to the erased background. This completes the first drum cycle.
At 6:00, during the second drum cycle (FIG. 4), step 5 occurs;
transfer media is gated along paper path 62 (FIG. 1) so that it
moves between the corona and the drum. Negative corotron 18 of
precharge/transfer station 32 charges the transfer media so as to
provide the electrostatic force which causes the toned image on
cyclindrical drum 12 to be transferred to the transfer media. The
transfer media for example, paper, is held against drum 12 by
electrostatic force only. In the preferred embodiment of the
invention, the same corotron current setting is used for both
precharge and transfer functions so that switching the current
level is not necessary except at the end of a multicopy run when
the unit must be turned off for the final clean cycle. Of course,
one alternative embodiment would be to switch the current setting
depending on whether the precharge function or the transfer
function is being performed.
At approximately 11:00 during the second drum cycle, step 6 occurs;
switching means 36 (FIG. 3), switches grid 24 so that the voltage
from third reference source 40 appears on the grid 24 so that the
charge on the photoconductor surface of rotating drum 12 is reduced
to a voltage near ground by the positive ions from the second bay.
In one embodiment the grid is set at approximately 0 volts. During
this cycle the negative, or first, bay of the dual bay corona has
negligible charging effect on the photoconductor due to the 0 volts
on the control grid. This change in voltage accomplishes the
preclean function.
At approximately 12:00 step 7 occurs; lamp 28 of interimage station
26 is energized to illuminate the entire photoconductor surface of
rotating drum 12 which discharges the photoconductor to
approximately 0 volts. At 1:00 during the second cycle, imaging
station 30 may be on or off. The photoconductor then rotates to
developer/cleaner station 60 where magnetic brush 58 removes
residual toner from the photoconductor surface. This completes the
two cycle process.
This unique configuration, as described above, has distinct
advantages over prior art configurations, in that the requirement
of high voltage power supply switching in short time intervals is
eliminated. In addition, the combination of the two corona units
requires one less power supply and one less corona unit for a
sizable cost reduction.
Another advantage of this configuration is the fact that the
transfer corona can be made smaller than would have been possible
if the combined charge and transfer coronas were used. This is
important in that significant reduction in the overall machine
dimension is achieved.
Still another advantage of the present invention is that the
overcharge for the photoconductor is more tightly controllable
which improves long term reliability of copy quality. The
electrostatic stress on the photoconductor is lessened since the
overcharge on the photoconductor is less than is required
heretofore.
The current density in the precharge/transfer corona is reduced
which significantly reduces the propensity for arcing.
While the invention has been particularly shown and described with
reference to a preferred embodiment thereof, it will be understood
by those skilled in the art that various changes in form and
details may be made therein without departing from the spirit and
scope of the invention.
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